In using IDL it is very easy to create too many variables to the point where
the computer memory capacity is exceeded.
It is possible to delete old
variables by using the DELVAR command. For example, if you wanted to delete
the variable DATA, you would type:
IDL> delvar, data
There is a peculiarity about using the IDL routine FINDFILE and using the
Unix symbol 
for the user's home directory. FINDFILE does not return any
files when using the command:
IDL> ff = findfile('/*')
It is recommended to use the full path (for example `/2p/morrison/*') instead
of .
It is possible to change the default directory from within IDL by using
the IDL command ``cd''. Some sample commands are:
IDL> cd, ''
IDL> cd, '/yd3'
The following will NOT work since it sets the default for the sub-process
and then exits that process.
IDL> $ cd /yd3 ;WILL NOT WORK
For Unix machines, it is possible to press control-z to place an
IDL session in the background so that you can do some work from the Unix
system. When you are ready to go back to the IDL session, type:
% fg
Be careful to remember that you have an IDL session hidden in the background
since you can waste memory and corrupt the color table settings if you
have multiple IDL sessions going.